1. Field of the Invention
The present invention relates to a wireless communications control technology and more particularly relates to a control technology for maintaining data communications efficiently in a wireless communications environment where a number of terminals share a limited band.
2. Description of the Related Art
In the field of wireless communications such as mobile telecommunications over a wireless LAN or a cellphone network, a number of terminals keep up data communications by sharing a certain wavelength band. In such a wireless communications environment, the average communication speed (i.e., the rate at which each of those terminals can get transmission or reception done) is usually lower than a situation where a single terminal exchanges data communications by itself. This is because if a number of terminals need to maintain data communications concurrently with each other, those terminals should share a limited band in accordance with the wireless communications access control method defined.
Hereinafter, a telecommunications technique using a wireless LAN will be described as a specific example.
FIG. 11 outlines the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidation) control system, which is an exemplary wireless LAN access control system. Suppose a situation where one wireless terminal 1111 is transmitting frame data (A1) 1112 to a base station 1101, while another wireless terminal 1121 is going to establish communication with the base station 1101. In that case, the wireless terminal 1121 cannot establish communication until the wireless terminal 1111 has transmitted the frame data (A1) 1112 completely. After the frame data (A1) 1112 has been transmitted, the wireless terminal 1121 starts to transmit another frame data (B1) 1122 to the base station 1101. And when the transmission of the frame data (B1) 1122 is complete, the wireless terminal 1111 starts to transmit next frame data (A2) 1113 to the base station 1101 again. In this manner, even if multiple terminals are attempting to keep up data communications concurrently, all wireless terminals other than the one that is communicating with the base station must wait until it gets ready to establish communication. As a result, the average communication speed of the wireless terminals will decrease, which is a problem.
Furthermore, in a wireless LAN, the link rate between the base station and each wireless terminal could vary according to the distance from the base station to the wireless terminal and/or the status of radio waves. For example, according to the IEEE 802.11g standard (which is one of wireless LAN telecommunications standards that were defined by the Institute of Electrical and Electronic Engineers in the United States), the link rate between a base station and each wireless terminal should be one of 54, 48, 36, 24, 18, 12, 9 and 6 Mbps, and the link rates could be different between any two of the wireless terminals. In that case, if one tries to establish communications at a relatively high link rate and at a relatively low link rate concurrently, then even the average data communication speed of a wireless terminal with the relatively high link rate would be determined by that of another wireless terminal with the relatively low link rate. That is also a problem.
To overcome such a problem, according to a conventional method, a band control technique is adopted. As for a wireless LAN, for example, the IEEE 802.11e standard was set up to define how to implement the QoS (Quality of Service) technique for ensuring a certain communication speed by reserving a band for a particular type of communications. By using a combination of a base station and wireless terminals that are compliant with the IEEE 802.11e standard, data communications between the base station and a particular one of the wireless terminals can be controlled preferentially. Another conventional technique proposed uses a base station that has the ability to exchange data with a particular wireless terminal while controlling access by the other wireless terminals with lower link rates than the current one (see Japanese Patent Application Laid-Open Publication No. 2007-74193, for example).
According to a wireless LAN implementation that uses a base station to which such a band control technique has been introduced in advance, however, normally only limited band control functions are available, among which not every wireless LAN user can find his or her desired band control function. Also, if a base station to which none of those band control functions available is introduced is used, then such a base station cannot provide the band control function that the user of the wireless LAN needs.
For example, suppose the user needs such a band control function for controlling the sum of the amounts of time it has taken for multiple wireless terminals, all of which have requested to establish data communication in a wireless LAN communications network that does not require real-time communications, to get their communications done. FIGS. 12A and 12B illustrate the amount of time it has taken for each of wireless terminals #1 and #2 to get data communications with the base station done by itself. Specifically, it has taken an amount of time Ta for the wireless terminal #1 to get frame data A1 through A4 transmitted completely, while it has taken an amount of time Tb for the wireless terminal #2 to get frame data B1 through B4 transmitted completely. In this case, the minimum sum of those amounts of time taken is Ta+Tb, theoretically speaking.
Suppose these two wireless terminals have the same data communication rate and require the same amount of collision avoidation process time by CSMA/CA control for the sake of simplicity. Also, each of those frame data A1 through A4 and B1 through B4 to transmit is supposed to have the same data size and the wireless terminals #1 and #2 are supposed to have link rates of 48 Mbps and 24 Mbps, respectively.
If their actual communication speeds are as high as their link rates, the amount of time Tb it will take for the wireless terminal #2 to get the frame data B1 through B4 transmitted will be exactly twice as long as the amount of time Ta it will take for the wireless terminal #1 to get the frame data A1 through A4 transmitted. FIG. 12C shows the total amount of time it will take for these two wireless terminals to establish data communications with a base station with no band control capabilities in a situation where the communications need to be done on an equal opportunity basis. In that case, the frame data A1 will be transmitted first, the frame data B1 next, and then those two wireless terminals will alternately communicate with the base station on a frame data basis. That is why the total amount of time Tb1 it will take until the wireless terminal #2 gets the last frame data B4 transmitted will be Ta+Tb. Meanwhile, the total amount of time Ta1 it will take until the wireless terminal #1 gets the last frame data A4 transmitted is obtained by subtracting the amount of time it will take to transmit the frame data B4 from the total amount of time Tb1. Consequently, the sum of those amounts of time Ta1+Tb1 is given by (Ta+Tb)+(Ta+Tb−the amount of time it takes to transmit the frame data B4). Thus, a band control technique for reducing that total amount of time to the theoretical minimum sum Ta+Tb is required.
In the case of wireless LAN communications, even if the communications started at a constant link rate, the link rate could still change after that during the communications due to a variation in the status of radio waves, for example. That is why if the user needs a band control technique for keeping up communications using a wireless terminal with the highest link rate preferentially, then the wireless terminals to which a top priority is given should be changed during the communications. Also, even though some user may want to maintain communications until a wireless terminal that has once opened up a communication gets that communication done, another user may want to interrupt such a communication that has been once opened up by some wireless terminal but that has not been done yet and switch to a communication by another wireless terminal with a higher priority.
Furthermore, in some situation, the user may happen to change his or her mind and want to switch from the band control technique for minimizing the total amount of times it will take for multiple wireless terminals to get their communications done into the one for giving a top priority to communications by a particular wireless terminal.
As described above, if the band control technique required may be different from one user to another, it is usually difficult to get a base station in which all of those different band control functions are incorporated in advance.